Within the VTA is a large network of GABAergic interneurons that are connected together via gap junctions. Under normal circumstances, these neurons release appropriate amounts of GABA, which acts to control the release of dopamine in the brain (primarily in the nucleus accumbens), and maintain it at the right levels to keep the person feeling motivated and happy, but not too happy.

However, these GABA neurons have μ-opioid receptors, so when a μ agonist drug such as morphine or codeine is present, the GABA neurons are inhibited and thus the dopaminergic neurons are dis-inhibited, leading to the release of massive amounts of dopamine and the feeling of a "high" that can ultimately produce addiction.